In situ molecular contaminant scattering in the vacuum ultraviolet.

摘要

Molecular contamination of optical surfaces from outgassed material has been shown in many cases to proceed from accumulation centers, and to produce many roughly hemispherical "islands" of contamination on the surface. The starting conditions for forming these centers at the surface are not well understood, but each island appears to grow faster than the level of contamination is increasing on the surrounding surface area. In these cases the surface will appear on the microscopic level to be comprised of many small, roughly hemispherical islands of accumulated material with plains of relatively clean surface in between. This is analogous to the common static situation for scattering of near infrared light from surfaces contaminated by dust particles. The mathematics of the hemispherical scattering is simplified by introducing a virtual source below the plane of the mirror, allowing the use of Mie theory to produce a solution for the resulting sphere in transmission. This novel approach explains the resonance peak in scattered light and is observed using a fixed wavelength monitoring the scattered light as these islands grow in time. The real time measurement of scattering from these islands of contamination is first reported in this work.; Vacuum Ultraviolet (VUV) wavelengths were used to measure both the angularly scattered light and the extinction of specular light from optical surfaces in this research. These measurements were taken in a facility designed to record specular reflectivity and scattered intensities, in situ, while the surface underwent contamination due to outgassed molecular contaminants. In addition, bi-directional reflectance distribution function (BRDF) measurements both before and after the contamination were done using UV, VUV and visible light. Results of the measurements are presented and comparisons are made with a computer model of the scattering, which was developed as part of this research.